X wcfs: Give more time to stop wcfs server

10 seconds is apparently not enough on a busy machine: with it I'm seeing
several failures where glog is stuck in fsync flushing its buffers, for
example:

https://nexedijs.erp5.net/#/test_result_module/20210817-E9535884/25
https://nexedijs.erp5.net/#/test_result_module/20210817-130D3B7A2/23
https://nexedijs.erp5.net/#/test_result_module/20210817-130D3B7A2/21
...

let's try to double the timeout.

wcfs/__init__.py

@@ -386,7 +386,7 @@ def __init__(wcsrv, mountpoint, proc, ffuseabort):
 @func(Server)
 def stop(wcsrv, ctx=None):
     if ctx is None:
-        ctx, cancel = context.with_timeout(context.background(), 10*time.second)
+        ctx, cancel = context.with_timeout(context.background(), 20*time.second)
         defer(cancel)
     wcsrv._stop(ctx)